Achieving virgin equivalent quality from recycled paper is increasingly difficult as fiber from lower and lower grade sources is used. Curbside recycled paper is usually considered just about the lowest possible grade of paper being recycled today. One factor which seems to contribute to the difficulty of obtaining fiber having quality equivalent to virgin fiber from curbside recycled paper is the presence of large amounts of “stickies” and other contaminants in the paper; but industrial/commercial waste papers are also becoming increasingly contaminated, especially with troublesome adhesives. Stickies commonly result from pressure sensitive labels in the waste paper but can be created by other adhesives, plastic window envelopes and the like. During the papermaking process, stickies in the furnish cause problems both by precipitating out of the furnish onto machine parts as well as by remaining in the web where they contribute to dense spots of adhesive, ink, plastic, fiber and a host of other contaminants. Stickies are especially problematic when recycle fiber is being used for tissue products as, in bath and facial tissue, each ply of tissue may only have a thickness equivalent to perhaps seven to ten layers of fiber, so the dark dense spot resulting from a stickie which finds its way into the finished sheet detracts both from the aesthetic appearance of the sheet and its functional integrity. Stickies that deposit out on machine clothing interfere with the proper movement of water and/or air through the fabric, again potentially contributing to defects in the finished product.
As waste paper is converted into pulp which is usable in making tissue and towel products, it is subjected to many of the same processes used with virgin pulp. In fact many mills will use a combination of virgin and recycle pulp to “dial-in” quality. This can lead to difficulties in fiber processing as many of these procedures are relatively sensitive to throughput variations. An especially important process is “screening” in which pulp is fed through a “screen” to remove dirt and other contaminants. Virtually all of the pulp fed to a papermachine will pass through a series of screens, each configured to remove contaminants progressively smaller in size than the screen before. Each screen will typically also have some means for removing contaminants lodged on the screen so that the screen does not become blocked. The term “screen” is often used both to refer to the overall apparatus used for removing impurities from pulp and for the actual perforate structure which forms the heart of that apparatus. In this application, the aggregate of those structures that form the removable perforate structure are typically referred to as the screen basket. This application relates to the perforate screen structure and more particularly to the shape of the surface of the screen structure which contacts the fiber suspension from which impurities are to be removed. Typically, foils moving relative to the screen basket are used for this purpose. These foils are hypothesized to induce negative pressure pulses that clear debris off of the screen basket so that it can pass out of the screen through a reject line.